Answer:
Given: A triangle ABC in which ∠ CAB=∠2, ∠AC B=∠1,∠C BA=∠3. Also line X passes through vertex C forming three interior angles ∠1,∠4,∠5 such that AB║X C.
To prove:m∠1+m∠2+m∠3=180°
Proof: In Δ ABC,AB║X C
∠2=∠5 [∠2 and ∠5 are alternate angles, AB║X C, and AC is a Transversal ] ..............(1)
∠3=∠4 [∠3 and ∠4 are alternate angles ,AB║X C, and BC is a Transversal ]......................(2)
∠1,∠4 and∠5 lie on line XC.
∠1+∠4 +∠5=180°[ Linear pair axiom]
∠1+∠2+∠3=180° [ Using 2 and 3]
Hence proved.